Polymers from vegetable oils have obtained increasing attention as public policy makers and corporations alike have been interested in replacing traditional petrochemical feedstocks due to their environmental and economic impact. In recent years, the cost of renewable source derived monomers has become highly competitive (and in many cases more economical than petrochemical feedstocks). For example, with appropriate modification of soybean oil (such as conjugation of triglycerides, or development of soybean oil types that are particularly suitable for polymerization), the chemical properties, thermal properties, microstructure and morphology, and mechanical/rheological behaviors of soybean oil-based polymers could be fine-tuned to make these biopolymers highly useful in the plastics industry.
To date, some success has been achieved through the application of traditional cationic and free radical polymerization routes to vegetable oils to yield thermoset plastics. Pfister & Larock, Bioresource Technology 101:6200(2010), which is hereby incorporated by reference in its entirety, have researched a variety of polymers, ranging from soft rubbers to hard, tough plastics using cationic copolymerization of vegetable oils, mainly SBO, to produce thermoset plastics with boron triflouride diethyletherate (BFE) as the initiator. Lu et al. synthesized soybean-oil-based waterborne polyurethane films with different properties ranging from elastomeric polymers to rigid plastics by changing the polyol functionality and hard segment content of the polymers (Lu et al., Polymer 46:71 (2005); Lu et al., Progress in Organic Coatings 71:336 (2011), which are hereby incorporated by reference in their entirety). Bunker et al. have reported the use of soybean oil to synthesize different bio-based products such as sheet molding composites, elastomers, coatings, foams, etc. For instance, Bunker et al. were able to synthesize pressure-sensitive adhesives using mini-emulsion polymerization of acrylated methyl oleate, a monoglyceride derived from soy bean oil (Bunker et al, International Journal of Adhesion and Adhesives 23:29 (2003); Bunker & Wool, Journal of Polymer Science Part A: Polymer Chemistry 40:451 (2002), which are hereby incorporated by reference in their entirety). The polymers produced were comparable to their petroleum counterparts.
Zhu et al. were able to generate an elastic network based on acrylated oleic methyl ester through bulk polymerization using ethylene glycol as the crosslinker (Zhu & Wool, Polymer 47:8106 (2006), which is hereby incorporated by reference in its entirety). Lu et al. were also able to create thermosetting resins synthesized from soybean oil that can be used in sheet molding compound applications. These resins were synthesized by introducing acid functionality and onto the soybean. The acid groups reacted with divalent metallic oxides or hydroxides forming the sheet, while the carbon-carbon groups are subject to free radical polymerization (Lu et al., Polymer 46:71 (2005), which is hereby incorporated by reference in its entirety). Bonnaillie et al. were able to create a thermoset foam system using a pressurized carbon dioxide foaming process of acrylated epoxidized soybean oil (AESO) (Bonnaillie & Wool, Journal of Applied Polymer Science 105:1042 (2007), which is hereby incorporated by reference in its entirety). Wool et al. were able to synthesize liquid molding resins that were able to be cured into high modulus thermosetting polymers and composites using triglycerides derived from plant oils (U.S. Pat. No. 6,121,398 to Wool et al., which is hereby incorporated by reference in its entirety).
Block copolymers may be thermosetting or thermoplastic with broad areas of application including as rubbers or elastomers; as toughened engineering thermoplastics; as asphalt modifiers; as resin modifiers; as engineering resins; as leather and cement modifiers; in footwear, such as in rubber shoe heels, rubber shoe soles; in automobiles, such as in tires, hoses, power belts, conveyor belts, printing rolls, rubber wringers, automobile floor mats, mud flaps for trucks, ball mill liners, and weather strips; as adhesives, such as pressure sensitive adhesives; in aerospace equipment; as viscosity index improvers; as detergents; as diagnostic agents and supports therefore; as dispersants; as emulsifiers; as lubricants and/or surfactants; as paper additives and coating agents; and in packaging, such as food and beverage packaging materials.
Styrenic block copolymers (SBCs), such as styrene-butadiene type polymers (e.g., styrene-butadiene di-block, SB; styrene-butadiene-styrene tri-block, SBS) of the type sold by Kraton Performance Polymers, Inc. under the Kraton® mark, have historically served the asphalt and footwear industries for years, with markets also in the industries of packaging, pressure sensitive adhesives, packaging materials, pressure sensitive adhesives, tires, packaging materials, footwear, and as a modifier of bitumen/asphalt, which is one of its largest markets.
With the forecast of increasing demand of liquid asphalt for the next decade, a particularly strong need exists for a new type of cost effective, environmentally friendly, viable polymer that can be used as an asphalt modifier in lieu of standard styrene-butadiene type modifiers. The global asphalt market is predicted to reach 118.4 million metric tons by 2015, according to a January 2011 report by Global Industry Analysts, Inc. The asphalt paving industry accounts for the largest end-use market segment of asphalt. With increasing growth in the developing markets of China, India, and Eastern Europe, asphalt will be increasingly needed to construct roadway infrastructure for the next decade. The increased demand for asphalt, along with the need for improved asphalt materials/pavement performance, creates the opportunity for an asphalt modifier.
In this regard, as background, the grade of the asphalt governs the performance of paving mixtures at in-service temperatures. In many cases, the characteristics of bitumen need to be altered to improve its elastic recovery/ductility at low temperatures for sufficient cracking resistance as well as to increase its shearing resistance for sustained loads and/or at high temperatures for rutting resistance. The physical properties of bitumen are typically modified with the addition of SBS polymers to produce an improved asphalt grade that enhances the performance of asphalt paving mixtures. Of the asphalt mixtures that are polymer modified, approximately 80% of polymer modified asphalt uses SBS-type polymers.
Asphalt cement is commonly modified with poly (styrene-block-butadiene-block-styrene) (SBS), a thermoplastic elastomer (TPE). Polymer modification is known to substantially improve the physical and mechanical properties of asphalt paving mixtures. Polymer modification increases asphalt elasticity at high temperatures, as a result of an increased storage modulus and a decreased phase angle, which improves rutting resistance. It also increases the complex modulus, but lowers creep stiffness at low temperatures, thus improving cracking resistance (Isacsson & Lu, “Characterization of Bitumens Modified With SEBS, EVA and EBA Polymer,” Journal of Materials Science 4:737-745 (1999), which is hereby incorporated by reference in its entirety). SBS-type polymers are typically added to asphalt pavements when additional performance is desired or when optimizing life-cycle costs is warranted. SBS allows for the production of many specialty mixes including cold mixes, emulsion chip seals, and micro-surface mixes.
SBSTPEs are block copolymers (BCPs) comprised of styrene-butadiene-styrene polymer chains that create an ordered morphology of cylindrical glassy polystyrene block domains within a rubbery polybutadiene matrix (Bulatovic et al., “Polymer Modified Bitumen,” Materials Research Innovations 16(1):1-6(2012), which is hereby incorporated by reference in its entirety). SBS polymers are thermoplastic, meaning that they can be easily processed as liquids at temperatures higher than their glass transition temperature due to the linear nature of their chains. Upon cooling, the rigid polystyrene end-blocks vitrify and act as anchors for the liquid rubbery domains by providing a restoring force when stretched (FRrEn J. R., Polymer Science and Technology (Prentice Hall, Upper Saddle River, N.J., 2 ed. 2008), which is hereby incorporated by reference in its entirety).
SBS is incorporated into asphalt through mixing and shearing at high temperatures to uniformly disperse the polymer. When blended with asphalt binder, the polymer swells within the asphalt maltene phase to form a continuous tridimensional polymer network (Lesueur, “The Colloidal Structure of Bitumen: Consequences on the Rheology and on the Mechanisms of Bitumen Modification,” Advances in Colloid and Interface Science 145:42-82 (2009), which is hereby incorporated by reference in its entirety). At high temperatures, the polymer network becomes fluid yet still provides a stiffening effect that increases the modulus of the mixture. At low temperatures, a crosslinked network within the asphalt redevelops without adversely affecting the low temperature cracking performance due to the elastic properties of the polybutadiene (Airey G. D., “Styrene Butadiene Styrene Polymer Modification of Road Bitumens,” Journal of Materials Science 39:951-959 (2004), which is hereby incorporated by reference in its entirety). The resulting performance properties widen the working temperature range of the binder-polymer system.
The butadiene monomer used in SBS is typically derived from petrochemical feedstocks, a byproduct of ethylene production. Unfortunately in light of the aforementioned growth in demand for liquid asphalt, however, the price of butadiene has been rapidly increasing not only due to increases in the price of crude oil, but also due to global market shifts in supply and demand. As shale gas supplies become more abundant, crackers are more commonly using lighter petrochemical feeds such as ethane to produce ethylene and its co-products. However, using lighter feeds lowers butadiene production, thus tightening the supply (Foster, “Lighter Feeds in US Steam Crackers Brings New Attitude Toward On-purpose Butadiene, Propylene Prospects,” Platts Special Report: Petrochemicals 1-6 (2011), which is hereby incorporated by reference in its entirety).
As briefly summarized above, vegetable oils have been considered as monomeric feedstocks for the plastics industry in general for over 20 years. To date, moderate success has been achieved through the application of traditional cationic and free radical polymerization routes to vegetable oils to yield thermoset plastics (i.e., plastics which, once synthesized, permanently retain their shape and are not subject to further processing). However, the vast majority of commodity polymers are highly processable thermoplastic materials, and the body of work related to the development of vegetable oil-based alternatives to conventional monomers like butadiene is much more limited.
Recently published US 2013/0184383 A1 to Cochran et al., “Thermoplastic Elastomers Via Atom Transfer Radical Polymerization of Plant Oil” (hereafter, the “'383 Cochran et al.” or “Cochran et al.” application, hereby incorporated by reference in its entirety together with any publications incorporated by reference in turn by Cochran et al.), however, describes novel thermoplastic elastomer compositions from vegetable oil monomers and methods of making and using the same. In particular, thermoplastic block copolymers are described in the '383 Cochran et al. application which comprise a block of radically polymerizable monomer and a block of polymerized plant oil containing one or more triglyceride monomers.
In Cochran et al., the block copolymers in question are summarized as comprising at least one PA block and at least one PB block. The PA block represents a polymer block comprising one or more units of monomer A, and the PB block represents a polymer block comprising one or more units of monomer B. Monomer A is described as a vinyl, acrylic, diolefin, nitrile, dinitrile, or acrylonitrile monomer, while Monomer B is a radically polymerizable plant oil monomer containing one or more triglycerides. The vegetable oil triglycerides are discrete monomers comprising three fatty acid chains esterified to a glycerol backbone.
Cochran et al. as well contemplates a method of preparing a thermoplastic block copolymer or polymer block wherein a radically polymerizable plant oil monomer containing one or more triglyceride monomers is initially provided. This plant oil monomer is then radically polymerized, in the presence of an initiator added as a separate component and a transition-metal catalyst system to form a thermoplastic polymer. This thermoplastic polymer can itself be used as a thermoplastic elastomer, or can be used as a thermoplastic polymer block and further polymerized with other monomers to form a polymerized plant oil-based thermoplastic block copolymer.
The addition of styrene to polymerized triglycerides helps improve the processability, aids in the control of the melt state properties of polymers (glass transition temperature (Tg), elastic moduli, etc.) (Woof, R. P. & Sun, X. S., Bio-based polymers and composites (Academic Press, Burlington, Mass. 2005), which is hereby incorporated by reference in its entirety), and can serve as physical crosslinking sites below the glass transition temperature (Tg) of the polystyrene (100° C.). In a typical SBS elastomer, the styrene composition is about 10-30 wt % such that spherical or cylindrical styrene domains form in a matrix of butadiene. When the temperature is below the glass transition temperature of polystyrene (T=100° C.), the polybutadiene matrix is liquid but is bound between the vitreous polystyrene spheres, which serve as physical crosslinks. When the temperature is above the glass transition temperature of polystyrene, the entire elastomer system is molten and may be processed easily. Crosslinked poly(soybean oil) has been reported to have T values as low as −56° C. (Yang et al., Journal of Polymers and the Environment 19:189 (2011), which is hereby incorporated by reference in its entirety). Accordingly, polymerized soybean oil is an excellent candidate to serve as the liquid component in thermoplastic elastomers based on styrenic block copolymers, and polymers based on radically polymerizable renewable source-derived polymer oil macroinitiator containing one or more polymer oils comprise a significant improvement due to their crosslinked nature.